Abstract [en]

Ash related problems have more than occasionally been observed in pellet burners during the last years. These problems can lead to reduced accessibility of the combustion systems as well as bad publicity for the market. The objectives of the present work were to; (i) evaluate how different raw materials for pellets affect the accessibility of the existing burner equipment, (ii) determine which of the ash forming element(s) that could be responsible for the deposit/slagg formation and, (iii) estimate the critical slagging temperature for the different raw materials. Stored and fresh materials from sawdust, logging residues and bark were used as raw material in three different pellet burners. The results showed that the slagging properties were relatively sensitive to the variations in total ash content and ash forming elements of the fuel. It is therefore recommended that ash rich fuels like bark and logging residues should not be used in the existing residential pellet burners. Both fuel and burner type affected the amounts of ash deposit produced. The degree of sintering (i.e. the strength of the deposits) was mostly affected by the fuel composition. Subsequent controlled sintering test of the produced deposits/slags showed critical slagging temperatures of about 850–900 °C for stored bark and about 1000 °C for fresh bark and stored and fresh materials from sawdust and logging residues. The results further indicated that the Si-content in the fuel correlated (relatively) well to the sintering tendencies in the burners. Chemical equilibrium models were used to interpret the experimental findings, and good quantitative agreements between modelling and experimental results were generally obtained.

Boman, Christoffer

Abstract [en]

Biomass is considered to be a sustainable energy source with significant potentials for replacing electricity and fossil fuels, not at least in the residential sector. However, present wood combustion is a major source of ambient concentrations of hydrocarbons (e.g. VOC and PAH) and particulate matter (PM) and exposure to these pollutants have been associated with adverse health effects. Increased focus on combustion related particulate emissions has been seen concerning the formation, characteristics and implications to human health. Upgraded biomass fuels (e.g. pellets) provide possibilities of more controlled and optimized combustion with less emission of products of incomplete combustion (PIC´s). For air quality and health impact assessments, regulatory standards and evaluations concerning residential biomass combustion, there is still a need for detailed emission characterization and quantification when using different fuels and combustion techniques.

This thesis summarizes the results from seven different papers. The overall objective was to carefully and systematically study the emissions from residential biomass combustion with respect to: i) experimental characterization and quantification, ii) influences of fuel, appliance and operational variables and iii) aspects of ash and trace element transformations and aerosol formation. Special concern in the work was on sampling, quantification and characterization of particulate emissions using different appliances, fuels and operating procedures.

An initial review of health effects showed epidemiological evidence of potential adverse effect from wood smoke exposure. A robust whole flow dilution sampling set-up for residential biomass appliances was then designed, constructed and evaluated, and subsequently used in the following emission studies. Extensive quantifications and characterizations of particulate and gases emissions were performed for residential wood and pellet appliances. Emission factor ranges for different stoves were determined with variations in fuel, appliance and operational properties. The emissions of PIC´s as well as PMtot from wood combustion were in general shown to be considerably higher compared to pellets combustion. PAHtot emissions were determined in the range of 1300-220000 µg/MJ for wood stoves and 2-300 µg/MJ for pellet stoves with phenantrene, fluoranthene and pyrene generally found as major PAH´s. The PM emissions from present residential appliances was found to consist of significant but varying fractions of PIC´s, with emissions in the range 35-350 mg/MJ for wood stoves compared to 15-45 mg/MJ for pellet stoves. Accordingly, the use of up-graded biomass fuels, combusted under continuous and controlled conditions give advantageous combustion conditions compared to traditional batch wise firing of wood logs. The importance of high temperature in well mixed isothermal conditions was further illustrated during pellets combustion to obtain complete combustion with almost a total depletion of PIC´s. Fine (100-300 nm) particles dominated in all studied cases the PM with 80-95% as PM1. Beside varying fractions of carbonaceous material, the fine PM consisted of inorganic volatilized ash elements, mainly found as KCl, K3Na(SO4)2 and K2SO4 with mass concentrations at 15-20 mg/MJ during complete combustion. The importance of the behavior of alkali elements for the ash transformation and fine particle formation processes was further shown, since the stability, distributions and compositions also directly control the degree of volatilization. In addition to the alkali metals, zinc was found as an important element in fine particles from residential biomass combustion. Finally, the behaviour of volatile trace elements, e.g. Zn and Cd, during pellets production and combustion were studied. A significant enrichment in the pellet fuel during the drying process was determined. The magnitude and importance of the enrichment was, however, relative small and some alternative measures for prevention were also suggested.